Calcium and magnesium metabolism during long-term treatment with thiazides

The effects of treatment with bendroflumethiazide (2.5 mg twice daily with potassium supplements) on mineral metabolism were evaluated in 111 patients followed for a minimum period of one year. The urinary calcium decreased with approximately 30% irrespective of the pretreatment levels and remained on this lower level during follow-up. This reduction of urinary calcium was associated with a reduction of the intestinal calcium absorption. The serum calcium increased slightly in most of the patients and in 10% of them hypercalcaemia could be demonstrated at some time. In most of these cases the hypercalcaemia was slight and temporary and when sustained rapidly reversed upon withdrawal of therapy. Initially the urinary magnesium excretion increased but after two years' treatment the values were no longer raised. The serum magnesium levels showed a continued decrease during follow-up but in muscle biopsies, performed after three years treatment, no magnesium deficiency was evident. During treatment also the fasting urinary calcium was reduced, suggesting reduced bone resorption, but urinary hydroxyproline was unchanged. The serum PTH levels appeared unchanged during follow-up. Thiazides appear to cause a persisting reduction of calcium excretion, which is compensated by reduced intestinal uptake, whereas parathyroid function is unaffected. Although there was a gradual decline of serum magnesium, magnesium deficiency was not demonstrated.